HIV assembles in a two step process in which an immature virion composed of the Gag polyprotein assembles at the plasma membrane, acquires the envelope glycoprotein, and buds from the infected cell. In the second step, a viral encoded protease cleaves the Gag polyprotein into its constituent matrix (MA), capsid (CA), and nucleocapsid (NC) structural domains. Cleavage results in a profound morphological rearrangement of th structural domains marked by the formation of a conical core of CA surrounding a complex of the NC and viral RNA. If the core is not well formed, either through blocked cleavage or the introduction of mutations, the resultant virus is non-infectious. This suggest that blocking the structural rearrangement is a potential therapeutic approach. To do so requires a detailed understanding of the immature and mature virions as well as the sequence of events driving the transformation. We have developed a mass spectrometry based hydrogen/deuterium exchange and crosslinking approach that allows us to the structural rearrangements that accompany maturation at the molecular level. In this application we propose to use that technology to: Aim 1- Determine whether CA exists in a domain swapped configuration in either the immature or mature virion Aim 2- Obtain a detailed understanding of the process of HIV maturation at the molecular level Aim 3 - Assemble stable hexamers of HIV-1 CA and peform a detailed biophysical and structural analysis to fully characterize the N-terminal domain/C-terminal domain interaction that is formed upon maturation.